1. Field of the Invention
The present invention generally relates to a memory, in particular, to a phase change memory.
2. Description of Related Art
With the continuous progressing and evolution of the semiconductor technology, the manufacturing process of memory devices also strides forward towards physical limits. Thanks to the advantages of small volume, low power consumption, high read/write speed, high capacity density, and the like, a phase change memory (PCM) is considered as one of non-volatile memory devices that are developed with great efforts currently.
Generally speaking, the PCM takes a chalcogenide (Ge—Sb—Te) as storage media. The chalcogenide changes between two phases of amorphous and crystalline phases under different annealing temperature conditions, and the two phases have different resistances. Therefore, the high-resistance amorphous phase and the low-resistance crystalline phase may be used for the memory to store digital data of “0” and “1”. Particularly, the two phases of the chalcogenide is reversible, so that the PCM can be repeatedly used for performing the operations such as programming, reading, and erasing.
In a conventional diode type PCM, a plurality of memory cells is disposed on a buried word line, and metal silicide layer is formed on the buried word line between memory cells. In other words, the metal silicide layer disposed on the word line is discontinuous. Therefore, when a current flows from a memory cell to a signal contact, a transmission path thereof not only includes the metal silicide layer, but also includes a substrate below the plurality of memory cells where the current flows. The resistance of the metal silicide layer is much smaller than that of the substrate, thus an impedance difference of the transmission paths between each of the memory cells on the same word line and the signal contact is rather large, which results in a large current difference and a large voltage difference between the memory cells when the data is read from or written into the memory cells on the same word line. And, the large current difference and the large voltage difference between the memory cells result in data misjudgment when a data is written in each of the memory cells or data reading error when a data is read from each of the memory cells.